Add firstToUpper and firstToLower to change first character's case

I use this sometimes and it would be nice to have in String.Extra I think

``` elm
firstToUpper : String -> String
firstToUpper str =
String.uncons str
|> Maybe.map (Tuple.mapFirst Char.toUpper)
|> Maybe.map (\( c, rest ) -> String.fromChar c ++ rest)
|> Maybe.withDefault str

firstToLower : String -> String
firstToLower str =
String.uncons str
|> Maybe.map (Tuple.mapFirst Char.toLower)
|> Maybe.map (\( c, rest ) -> String.fromChar c ++ rest)
|> Maybe.withDefault str
```